Such devices are used for example to provide displays in radar equipment. At the instant each radar pulse is transmitted, the CRT team begins to be deflected away from the centre of the tube face radially towards its periphery. The received echo pulse is arranged to amplify the beam so that a bright spot appears on the tube face, at a radial distance from its centre proportionate to the distance of the object which caused the echo. The deflection coils which deflect the CRT beam are continually rotated about the axis of the CRT by means of suitable and well known arrangements, in synchronism with rotation of the scanner from which the radar pulse is transmitted, and at which the reflected echo is received. Consequently, each successive beam scan is in a slightly different radial position, progressively incrementing round the tube face, with the result that a complete display of the radar-reflecting features located in all directions around the scanner is provided.
It is normally required, particularly in radar equipment used on boats, to be able to set the equipment for different ranges. That is to say, such that the picture on the tube face represents on a relatively large scale the radar-reflecting features present within a relatively short distance of the scanner, or represents on a smaller scale the features present within a greater distance of the scanner. This is achieved by radially deflecting the beam at different speeds for different ranges. The faster the deflection, the sooner the beam will arrive at the periphery of the tube face, and consequently the closer to the scanner will be any object which produces an image at the periphery of the tube face. Conversely, if the beam is deflected more slowly, that same object would produce its image closer to the tube centre, and a more distant object would produce an image at the tube periphery. Thus, for each range required, a corresponding beam deflection speed must be employed, the highest speed corresponding to shortest range and the lowest speed corresponding to the longest range.
In the prior art, in order to achieve this, a range inductor which has tappings at various points along its length is coupled to the deflection coils. A range selecting switch is operable so as to connect successively longer sections of the range inductor in series with the deflection coils, so that the total inductance of the active portion of range inductor and the deflection coils themselves successively increases as the range selecting switch is moved through its positions. To initiate each beam deflection, a scan coil driver circuit applies a constant voltage to this combined inductance and this results in a linear rise in the current flowing through the deflection coils and consequently a linear deflection of the CRT beam. The rate at which this ramp current rises is determined by the value of the total inductance, such that the beam deflection is fastest when the inductance is lowest i.e. when the smallest section, or none, of the range inductor is connected to the coils, and is slowest when the total inductance is greatest i.e. when the whole of the range inductor is connected to the coils.
Since the inductance of the coils is fixed, the whole range of inductance adjustment has to be accommodated by the range inductor. It must therefore be quite large and consequently relatively expensive and also, when operating at the longest ranges, a relatively large proportion of the total active inductance will be in the range inductor and a relatively lower proportion in the deflection coils themselves, so that a significant amount of power is wasted through being dissipated in the range inductor. This is significant, particularly in radar equipment intended for small boats, because in this situation the power generation and storage facilities available are small. The power consumption of radar equipment presently available makes it unsuitable for many small boats, and reduction of the power consumption of such equipment assists in making it practical for installation on small boats with limited power generation and storage facilities as, of course, do factors which reduce the cost of the radar equipment itself.